In a fuel cell system having a fuel cell that produces power through the electrochemical reaction between hydrogen and oxygen (air) for use as a drive source of a mobile object, such as a vehicle or a ship, if it is in a cold state at a temperature of 0° C. or lower, water present near the electrodes freezes, and could inhibit diffusion of reactant gas or reduce the electric conductivity of an electrolyte membrane. More specifically, if a fuel cell is activated in a cold environment at a temperature of 0° C. or lower, freezing could cause clogging of a reactant gas flow path and inhibit reactant gas (hydrogen and air) from flowing and reaching an electrolyte membrane, and consequently, the electrochemical reaction would not proceed even if the fuel cell is supplied with fuel gas, thereby resulting in being unable to activate the fuel cell. Furthermore, due to the condensation occurring in the reactant gas flow path and the resulting water freezing, the gas flow path could be blocked.
As a method for solving the above problem in the prior art, for example, JP laid-open patent publication No. 2005-71626 discloses, with regard to a fuel cell system used in a low-temperature environment, a technique of setting an output current to a maximum value upon activation in below-freezing conditions. This prior-art technique can facilitate self power generation in the fuel cell and shorten warm-up time.
Here, the above-mentioned JP laid-open patent publication No. 2005-71626 discloses, as a method for measuring the internal resistance of a fuel cell, measuring a high-frequency impedance (paragraphs 0076-0099). Since the internal resistance of a fuel cell has a strong relationship with the amount of water (hereinafter referred to as the “water content”) left in the electrolyte membrane of the cell, the water content of the fuel cell can be estimated by measuring an AC impedance.
As a method of estimating such water content, for example, JP laid-open patent publication No. 2003-86220 discloses a fuel cell system that obtains the complex impedance of a fuel cell when a limiting wave signal is applied to an output signal of the fuel cell while changing the frequency from a high frequency to a low frequency, and estimates the fuel cell water condition from a resistance component R1, which increases when the internal water content of the fuel cell is insufficient, and a resistance component R2, which increases when the internal water content is too large. It is disclosed that the resistance component R1 is measured by applying a high-frequency sine-wave signal and the resistance component R2 is measured by applying a low-frequency sine-wave signal.
As a similar technique, JP laid-open patent publication No. 2003-297408 discloses a fuel cell system configured to detect the water content of a measurement gas by referring to either the voltage or current of an electrochemical cell.
Also, JP laid-open patent publication No. 2005-209635 discloses a fuel cell power production control method that detects the water left in both an electrolyte membrane and a reactant gas flow path to control the humidity of reactant gas, thereby optimizing the water left in the electrolyte membrane during the continuation of the fuel cell operation, and can consequently reduce or eliminate scavenging time when stopping the fuel cell and improve start-up performance when re-starting the fuel cell.
Furthermore, JP laid-open patent publication No. 2005-332702 discloses a fuel cell diagnostic apparatus that measures, by using a DC/DC converter for increasing/decreasing the voltage of a fuel cell with n cells or cell modules, the voltage change of each cell, etc., when an alternating current of a predetermined frequency is applied to the fuel cell, thereby measuring the internal resistance thereof, and detects an error of each cell, etc., from the above internal resistance and reactance.
Here, the invention disclosed in JP laid-open patent publication No. 2005-71626 is also an invention made in order to prevent loss or deterioration of carbon in the anode due to the oxidation of the carbon that would occur when the cell voltage of the fuel cell voltage goes below a specific voltage value, and in order to avoid such situation, an output current is set to a maximum value to the extent that the cell voltage of the fuel cell does not go below the specific voltage.